The present invention is directed to microelectronic devices, especially field effect transistors (FETs), which utilize closed cage structures and/or derivatives thereof for transport and storage of electrons in logic and memory cells, respectively.
Miniaturizing and size reductions of microelectronic transistors, such as field effect transistors, especially metal oxide semiconductor field effect transistors (MOSFETs), to their theoretically predicted miniaturization limits is not a simple task. As device sizes are scaled down in order to improve performance and to increase function per unit area, numerous fundamental problems arise in obtaining acceptably functioning devices. For example, as device sizes approach dimensions in 100 nm range, numerous problems occur in achieving the characteristics needed in the highly integrated circuits common today. For logic devices, these include sub-threshold conduction effects, output conductance and power gain of the device. For volatile memories, such as dynamic random access memories (DRAMs), and for non-volatile memory devices, such as electrically erasable programmable random access memory devices (EEPROMs and FLASHs), these include sub-threshold conduction which leads to leakage of the stored charge or the loss of the clearly defined threshold.
Recently, it has been disclosed that nanocrystals made from silicon are useful in memory elements due to large electrostatic energy at low capacitance resulting in a Coulomb barrier that can be seen for single electrons and due to the discreteness of the occupation states (See Tiwari, et al., App. Phys. Letter 1996, 68, No. 10, 1377-1379. The Coulomb barrier can also be used to make a single electron transistor structure where the flow of electrons through small-sized regions is utilized for obtaining functions in logic. Although the nanocrystals are an attractive solution to the problem of scaling down transistor memories, they still have problems associated therewith. For example, the nanocrystals vary in size, and it has been found that the variation in size of nanocrystals affects and leads to variations in the Coulomb barrier, thereby limiting the injection of electrons and hence the voltage at which the injection of electrons takes place. Additional variations may also occur because the energy level of the eigenstates (the energy levels of nanocrystals) is also affected by the size of the nanocrystals. This problem is minimized if a large number of nanocrystals (e.g., exceeding at least 100) are used for suppressing these statistical fluctuation effects observed with the nanocrystals, but this solution also constrains the smallest size device that can be made.
Thus, the search has been continuing for an improved means utilizing a different constituent than the nanocrystals that would permit further scaling down and avoid the fluctuation effects discussed hereinabove.
Obviously, this constituent is required to meet certain criteria. For example, it must be capable of exhibiting Coulomb blockade when placed in a semiconductor device.
Coulomb blockade is a phenomena that occurs at very small dimensions wherein the lowering of capacitance of a confined region in space creates the need for a reasonably large electrostatic energy (e2/2c) where e is the electron charge and c is the capacitance, equivalent to having an electrostatic barrier before an electron can be injected onto it. In other words, the electron is trapped or equivalently the flow of electrons is blocked as a result of the effect. Its central concept is that no current flows until the electron can charge a particle. For electrons to flow, they must pass from one material to another. This requires energy. The energy required to place an electron on a material is described by the equation:   E  =            1      /      2        ⁢                  e        2            c      
where E is the electrostatic energy required to place the electron on a material, c is the capacitance of the material, and e is the charge of the electron. Thus, as clearly seen by the equation, as the size of the capacitor becomes smaller, its capacitance also decreases, and the energy required to place an electron thereon becomes larger, i.e., the Coulomb blockade effect increases. Thus, the constituent used for the capacitor must exhibit this property. In fact, it is desirable that this constituent has a size or diameter of nanometer dimensions. Moreover, the size of the constituent should be reproducible and not variable, thereby avoiding the variations in the Coulomb barrier due to size fluctuations as seen with nanocrystals.
In addition, it is preferred that the constituent has highly reproducible structures with controlled dimensions and electrical properties. Moreover, the constituent should have reproducible electron reception and transport properties. Finally, it should exhibit electrical conductivity and be stable, especially during a change in temperature.
The present inventors have found molecular constituents that fulfill these requirements. These molecules have closed cage structures, e.g., FULLERENES, and their derivatives and the corresponding silicon molecules, e.g. Si28.
Accordingly, the present invention is directed to a microelectronic device in which a closed cage molecule is used. These molecules are nanometers in size. That is, the molecules have specific sizes. More specifically, the present invention is directed to a microelectronic device comprising a field effect transistor which comprises:
(a) a source and drain, the source and drain being comprised of a semi-conductor material doped with a first type of impurities;
(b) a channel extending from said source and drain and disposed therebetween, said channel comprised of said semi-conductor material doped with a second type of impurities;
(c) an insulating layer superimposed over said channel region, said insulating layer comprised of insulating material and at least one molecule having a closed cage structure, said molecule being capable of exhibiting Coulomb blockade and receiving and storing at least one electron emanating from either the source or drain or the channel upon application of sufficient voltage between said source and drain or channel and gate to overcome the Coulomb barrier; and
(d) a third layer comprising a gate over the insulating layer.
Another embodiment of the present invention is directed to a microelectronic device comprising a source and drain of a semiconductor material doped with a first type of impurities, a substrate comprising said semiconductor material doped with a second type of impurities, the insulating layer and the gate, whereby the substrate contains a recess therein which is approximately equal to the width or length of a molecule having a closed cage structure so that the latter is held in place and is in electrical contact with the source and the drain and separated from the gate by the insulating layer.
Another embodiment of the present invention is directed to a microelectronic device comprising a source and drain both of which are comprised independently of a doped semiconductor or metal, a channel containing a molecule having a closed cage structure which is in electrical contact with the source and drain and separated from the gate by an insulator.